Goodnight Moon.

I easily overlook the myriad plot holes in Harry Potter.  I enjoy the impossible science of Star Wars.  And who doesn't adore cartoon physics?  So why am I bothered that our moon had been destroyed in Tom Cruise's movie Oblivion?

Well, because it reminded me of a comment made by religious cult leader a while ago -  that the moon would leave us in the Messianic Era.

 If we lost our moon, temperature on earth would fluctuate extremely.  We would have deadly, burning summers and super-cold winters.  Humans would not survive.

The moon does not just revolve around the earth, the earth revolves around the moon.  Not exactly, the size differential causes the earth to just wobble.  If the two objects were of equal size, they would spin around each other.  In equation, as one gets larger than the other, its orbit gets smaller and smaller until it becomes a center of the other planet's circling.

This moon-caused wobbling of the earth makes the earth have a less elliptical orbit, more perfectly circular and even.  As the earth stretches away from the sun at its orbit apogee, the wobble holds it in; when the earth would come in closest to the sun, the wobble inertia keeps it moving a bit farther out.

Alternatively, sometimes its fun to make impossible alternative realities. On the detail of my book cover shown above, the two planets are not revolving around a sun.  This made for fun backpedaling of thought, of how this could be possible, in an alternative universe sense.

Below, I'm giving you a bonus image, a detail from another book cover, a follow up book to the above one.  (And on topic of missing moons!)